Typically, the optical systems used in the context of fabricating microelectronic devices such as semiconductor devices comprise a plurality of optical element units comprising optical elements, such as lenses and mirrors etc., in the light path of the optical system. Those optical elements usually cooperate in an exposure process to transfer an image of a pattern formed on a mask, reticle or the like onto a substrate such as a wafer. The optical elements are usually combined in one or more functionally distinct optical element groups. These distinct optical element groups may be held by distinct optical exposure units. In particular with mainly refractive systems, such optical exposure units are often built from a stack of optical element modules holding one or more optical elements. These optical element modules usually comprise an external generally ring shaped support device supporting one or more optical element holders each, in turn, holding an optical element.
Optical element groups comprising at least mainly refractive optical elements, such as lenses, mostly have a straight common axis of symmetry of the optical elements usually referred to as the optical axis. Moreover, the optical exposure units holding such optical element groups often have an elongated substantially tubular design due to which they are typically referred to as lens barrels.
Due to the ongoing miniaturization of semiconductor devices there is a permanent need for enhanced resolution of the optical systems used for fabricating those semiconductor devices. This need for enhanced resolution obviously pushes the need for an increased numerical aperture and increased imaging accuracy of the optical system.
One approach to achieve enhanced resolution is to reduce the wavelength of the light used in the exposure process. In the recent years, approaches have been made to use light in the extreme ultraviolet (EUV) range using wavelengths down to 13 nm and even below. In this EUV range it is not possible to use common refractive optics any more. This is due to the fact that, in this EUV range, the materials commonly used for refractive optical elements show a degree of absorption that is to high for obtaining high quality exposure results. Thus, in the EUV range, reflective systems comprising reflective elements such as mirrors or the like are used in the exposure process to transfer the image of the pattern formed on the mask onto the substrate, e.g. the wafer.
The transition to the use of high numerical aperture (e.g. NA>0.5) reflective systems in the EUV range leads to considerable challenges with respect to the design of the optical imaging arrangement.
Among others, the above leads to very strict requirements with respect to the relative position between the components participating in the exposure process. Furthermore, to reliably obtain high-quality semiconductor devices it is not only necessary to provide an optical system showing a high degree of imaging accuracy. It is also necessary to maintain such a high degree of accuracy throughout the entire exposure process and over the lifetime of the system. As a consequence, the optical imaging arrangement components, i.e. the mask, the optical elements and the wafer, for example, cooperating in the exposure process must be supported in a defined manner in order to maintain a predetermined spatial relationship between the optical imaging arrangement components as well to provide a high quality exposure process.
To maintain the predetermined spatial relationship between the optical imaging arrangement components throughout the entire exposure process, even under the influence of vibrations introduced via the ground structure supporting the arrangement and under the influence of thermally induced position alterations, it is necessary to at least intermittently capture the spatial relationship between certain components of the optical imaging arrangement and to adjust the position of at least one of the components of the optical imaging arrangement as a function of the result of this capturing process.
To deal with these problems, in common mainly refractive systems, the optical elements and the metrology devices necessary to capture the spatial relationship mentioned above are substantially rigidly mounted to a so called metrology frame. Such a metrology frame, in general, is a heavy, generally plate shaped body. The metrology frame is supported on the ground structure via vibration isolating mechanism to reduce the influences of vibrations of the ground structure usually lying in the range of about 30 Hz. Furthermore, considerable effort is necessary to avoid thermally induced deformations of the metrology frame. Either the metrology frame has to be made of a generally expensive material with a very low coefficient of thermal expansion or an expensive temperature stabilization system has to be provided. Thus, in any case, the metrology frame is a very complex and, thus, expensive part of the system.
A further problem arising with the use of light in the EUV range lies within the fact that, for a system with a high numerical aperture, at least the mirror closest to the wafer is generally of considerable size by far exceeding the diameter of the last optical elements used in conventional refractive systems. This poses particular problems to the metrology of the wafer stage leveling system providing the position adjustment between the wafer and the optical projection system formed by the optical elements.
In conventional refractive systems, the leveling of the wafer, i.e. the adjustment of the position of the wafer along the optical axis of the optical projection system (often vertical and, thus, often referred to as the z-axis), is often provided using the measurement results of a metrology arrangement mounted to the metrology frame and projecting a beam of light onto the wafer at an oblique angle from a location close to the outer periphery of the last refractive optical element of the projection system. The measurement beam is reflected from the surface of the wafer at an oblique angle as well and hits a receptor element of the metrology arrangement at a location also close to the outer periphery of the last refractive optical element. Depending on the location where the measurement beam hits the receptor element, the location of the wafer with respect to the metrology arrangement may be determined.
In the conventional refractive systems with last refractive optical elements of relatively moderate diameters, reliable high precision measurement results may be achieved. However, due to the large size of the mirror closest to the wafer and the small distance between the mirror and the wafer, in a high numerical aperture EUV systems as outlined above, the angle between the measurement beam and the wafer becomes too small to obtain reliable high precision measurement results.
EP 1 182 509 A2 (by Kwan), the disclosure of which is incorporated herein by reference, discloses an imaging system, wherein the positioning of the mask relative to the optical projection system is provided using the measurement results of a mask metrology arrangement mounted in part to the housing of the projection system and in part to the mask table carrying the mask. Here, the mask table carries the reference elements of the metrology arrangement, i.e. reflectors for interferometry measurements or 2D-gratings for encoder measurements. While this solution eliminates the need for mounting components of the mask metrology arrangement to a metrology frame it still has the disadvantage that parts of the metrology arrangement are mounted to the housing of the optical projection system. Since the housing may be affected by thermally induced expansion effects altering the position of the optical elements received therein, it is necessary to account for these effects when positioning the mask table adding further complexity to the system thus rendered more expensive.